1. Field of the Invention
The present invention relates to a process for producing a bicycle frame made of a fiber-reinforced plastics (FRP), using resin-impregnated continuous-fibers (prepreg).
2. Description of the Related Art
In general, bicycle frames are produced by welding metal pipes of, for example, iron-based materials, aluminum and titanium, but recently, bicycle frames of FRP pipes using carbon fibers or aramide fibers are produced, to produce a higher-quality or to lighten the weight of the bicycle frame.
The structure of an FRP pipe bicycle frame comprises FRP pipes and metal joints or lugs bonded together by an adhesive, but FRP frames using FRP lugs are considered most preferable when attempting an optimum lightening of or obtaining required frame characteristics, e.g., mechanical strength and rigidity, best suited to a particular usage. Accordingly, various processes for forming FRP bicycle frames have been and are being used.
The processes for forming FRP bicycle frames can be classified into two groups, i.e., bonding FRP pipes and FRP lugs with an adhesive (divisional forming process), and a simultaneous forming of the entire frame (simultaneous forming process) (Plastic Age, June 1988, pp. 134-140), but in all of these processes, currently a metal member must be inserted in a lug at a part of a front fork stem or a rear wheel, etc., and connected with another member.
In the divisional forming process, the pipes and lugs of a bicycle frame are first separately made, thereby facilitating control of the thickness of the part at respective portions and allowing a lightening of the weight. Nevertheless, these separately made pipes and lugs must be bonded by an adhesive, and thus a high level bonding technique is required and it is difficult to obtain a sufficient strength at the bonded portions.
A sufficient strength can be obtained by the simultaneous forming process, as there are no bonded portions of the pipes and lugs. This process is also considered promising when designing fashionable products, due to the high degree of design freedom. Nevertheless, a very high level forming technique is required when simultaneously forming a bicycle frame having a complex three-dimensional structure, and the productivity is very low. Further, a precise formation is particularly difficult, and thus a post processing such as grinding or polishing is required, which reduces the productivity as well as the mechanical strength and rigidity by cutting the fibers, and raising problems in the design from the viewpoint of the mechanical strength or rigidity of the frame. Further, since the frame size cannot be adjusted by cutting the forming pipes, as for a conventional frame, a mold is required for the entire frame for each frame size, which increases manufacturing costs.
Generally, when a complex part of an FRP is to be formed, one or usually more bodies of a foamed material are first arranged inside, a prepreg of resin impregnated continuous fibers is laminated thereon, a mold is set on the laminated prepreg, and a heat treatment is carried out for casing.
A lamination or winding-on of a prepreg of resin impregnated continuous fibers may be adopted to form a lug having a desired shape, but the material thereof is generally expensive and the workability is low. A cheap foam is used for solving the above problems. Nevertheless, in such a conventional process, an already foamed material must be disadvantageously machined to a desired shape, and when one or more foamed bodies are to be arranged outside a joint portion of a frame part from the viewpoint of reinforcement or aesthetics, spaces are formed between the joint and the foamed body and between the foamed bodies and times and works are consumed to fill the spaces.